1. Technical Field
This invention relates to signal detection circuits and, more particularly, to the detection of a DC loop current in a telephone subscriber loop occurring in response to an off-hook condition at the telephone station set in response to a telephone ringing signal.
2. Description of the Prior Art
It has become common to supply subscribers with telephone service over subscriber loop carrier systems. The subscriber loop carrier system is a signal multiplexing system which supplies more than one telephone subscriber channel over a single pair of twisted wires. Such carrier systems are very useful in situations where the cost of providing metallic pairs for each subscriber is prohibitive. They are also useful in situations where it is desirable to defer the laying of new cable until sufficient demand develops, or until known demand levels are ascertained. A final application of subscriber loop carrier systems is to provide temporary service to a particular location, such as, for example, the site of a political or athletic event.
In such carrier systems, it is not possible to transmit the actual ringing signals over the carrier facilities, since ringing signals must be provided for a plurality of different subscribers at the remote end of the carrier system. In this situation, ringing signals are generated at the remote locations and selectively applied to the particular subscribers by control signals sent over the carrier system. It is desirable in such a situation that the ringing signals be provided by means of apparatus which is small in size and economical, and consumes little power, since such apparatus must be located in the remote location.
One of the ancillary features required with such remote ringing supplies is the ability to trip the ringing signal, i.e., terminate the ringing signal, when the called subscriber goes off-hook. When the subscriber telephone station set is placed in the off-hook condition, a direct current path is provided between the tip and ring conductors of the telephone loop. The off-hook condition can be detected from the direct current flow in the telephone loop. Difficulty arises, however, in detecting this relatively small direct current flow in the presence of extremely large alternating current ringing signals.
For purposes of minimizing circuit size and cost and maximizing circuit reliability, it is desirable to detect at the remote terminal the small direct current flow in the telephone loop with circuits that can be fabricated using integrated circuit techniques. Since, however, presently most integrated circuits can process only low-voltage and low-current signals, signals which include as one component the high voltage ringing signals, cannot be economically processed directly by an integrated circuit. Prior art ring-trip detectors, which can be fabricated in part as an integrated circuit, such as disclosed in U.S. Pat. No. 4,262,177 to Brolin, on April 14, 1981, require several high-voltage external and discrete components, such as high-voltage transistors and diodes, and a capacitor in order to process the high-voltage ringing signals on the loop.
The problem, then, is to detect the small direct current flow in the presence of the high-voltage ringing signals by means of low-voltage circuits that can be fabricated using integrated circuit techniques with a minimum number of external discrete components.